1. Field of the Invention
The present invention relates to a polishing method and a polishing apparatus for a workpiece, such as a semiconductor substrate, and also relates to a method of monitoring a substrate. More particularly, the present invention relates to a polishing method, a polishing apparatus, and a substrate monitoring method suitable for use in monitoring a surface condition of the workpiece with a monitoring sensor while swinging a top ring that holds the workpiece to be polished.
2. Description of the Related Art
Fabrication of a highly integrated semiconductor device entails fine interconnects and multilayer structure, which require a surface flatness of a semiconductor substrate (which will be hereinafter referred to as “substrate”). Chemical mechanical polishing (CMP) has been conventionally used to remove surface irregularities of the substrate to provide a flat surface thereof.
In the chemical mechanical polishing procedure, it is necessary to terminate polishing of the substrate at a desired film thickness. For example, there is a case where it is required to leave an insulating layer, such as SiO2, over metal interconnects, such as Cu (copper) or Al (aluminum), in order to form, in a subsequent step, another metal layer on the insulating layer, which is called an interlayer dielectric. In such a case, if polishing is performed more than necessary, sufficient insulation performance cannot be obtained. Therefore, it is necessary to terminate the polishing process so as to leave the interlayer dielectric with a predetermined film thickness.
In the device fabrication procedure, trenches for interconnects in predetermined patterns are formed on a substrate in advance, and the trenches are filled with Cu (or alloy thereof). Subsequently, unwanted portions of Cu on the surface of the substrate are removed by CMP. When polishing the Cu layer by CMP, it is necessary to selectively remove the Cu layer so as to leave Cu only in the trenches for interconnects. Specifically, it is required to remove the Cu layer in regions other than in the trenches until a barrier layer (composed of TaN, for example) is exposed.
Thus, a CMP apparatus typically includes a monitoring sensor for detecting and monitoring a polished condition of a substrate surface during polishing. An end point of the polishing process is determined based on measurements of the monitoring sensor.
It is known that a polishing profile is substantially axisymmetric with respect to an axis extending through a center of rotation of the substrate in a direction perpendicular to a surface to be polished, due to rotation of a top ring that holds the substrate. Therefore, it is important to detect and monitor the polished surface condition by the monitoring sensor in all radial positions on the substrate including a substrate center and substrate edges where some peculiarities, such as excessive polishing or insufficient polishing, are likely to occur.
FIG. 18 is a view showing a positional relationship between a polishing table 500 and a substrate 550 in a CMP apparatus. As shown in this figure, the CMP apparatus is configured to hold and rotate the substrate 550 by a top ring and bring the substrate into contact with a surface (i.e., a polishing surface) 501 of the rotating polishing table 500, thereby polishing a surface (i.e., a surface to be polished) of the substrate 550 uniformly. The monitoring sensor is mounted on the polishing table 500 at a predetermined location. Specifically, the monitoring sensor is situated at a predetermined point in a sensor locus 11 indicated by a dashed line in FIG. 18. The monitoring sensor detects the polished condition of the surface of the substrate 550 when the monitoring sensor is positioned under the substrate 550.
This CMP apparatus polishes the substrate 550 while rotating and oscillating the substrate 550 by swinging the top ring during polishing of the substrate 550. Specifically, the substrate 550 is moved between a position indicated by a solid line and a position indicated by a dotted line in FIG. 18. When monitoring the surface of the substrate 550 by the monitoring sensor, a rotation angle of the polishing table 500 (i.e., a rotation angle of the monitoring sensor) is detected in each revolution of the polishing table 500, so that signal from the monitoring sensor is monitored focusing on measuring points, i.e., black points al shown in FIG. 18, where the monitoring sensor moves under the substrate 550 every time the polishing table 500 makes one revolution regardless of the swinging position of the top ring.
In such a polishing procedure, however, the monitoring sensor cannot monitor all of the measuring points on the substrate (i.e., cannot monitor white points a2 in FIG. 18 and the center of the substrate when the substrate is located as indicated by the dotted line). Specifically, it is difficult to monitor the substrate center and the substrate edges, and stable monitoring data cannot be obtained.
Further, since the top ring is swung, the radial position of the measuring point with respect to the substrate surface varies every time the polishing table 500 makes a revolution, making it difficult to perform consistent and stable monitoring of the substrate during polishing. This is particularly problematic in a case of performing real-time controlling of a polishing profile based on the monitoring data, because it is necessary to grasp an accurate film-thickness profile in each radial position during polishing.